1. Field of the Invention
The present invention relates to a method of tempering glass product by a heat treatment. More particularly, it relates to a method of tempering glass by quenching the glass product heated at higher than a strain point and lower than a softening point by dipping it into a liquid coolant.
2. Disclosure of the Prior Art
It has been known to employ the method of tempering glass product by quenching the glass product heated at higher than a strain point and lower than a softening point by blowing a gaseous coolant such as air on the surface of the heated glass product to give high temperature difference between the surface and the central part of the glass product at the strain point whereby high compressive stress in the surface layer of the glass product could not give after quenching it to the room temperature.
However, the method is effective for a glass plate having a thickness of more than about 4 mm, and the method has been employed as the industrial method. However, when it is applied for a glass plate having a thickness of less than 3 mm, the quenching function for imparting enough temperature difference between the surface and the central part of the glass plate could not be given whereby high compressive stress could not be given in the surface layer of the glass plate.
It has been proposed as a method of tempering a glass plate having a thickness of less than 3 mm, to dip the glass plate heated at higher than a strain point and lower than a softening point into a liquid coolant in order to impart high temperature difference between the surface and the central part of the glass plate to give high compressive stress in the surface layer of the glass plate after quenching it to the room temperature. The method is referred as a tempering method by liquid quenching. The method has been disclosed in U.S. Pat. Nos. 3,640,694 and 3,679,388, etc.
Thus, in order to prevent the break of the glass by stress caused in the quenching in the tempering method by liquid quenching, it has been proposed to relieve the quenching function by forming a gas layer between the glass surface and the coolant as disclosed in U.S. Pat. No. 3,186,816 or to use a liquid coolant which has lower quenching function.
However, it has been difficult to obtain the glass product having satisfactory strength.
It has been porposed to stir a bath by a stirrer for imparting uniform temperature in the liquid coolant as disclosed in West German Pat. No. 640,176. It has been difficult to impart enough quenching in uniform by the method.
It has been also proposed to dip the glass plate heated at high temperature into a liquid coolant under ejecting the flow of coolant from an orifice in the liquid coolant to the glass plate as disclosed in Japanese Unexamined Patent Publication No. 99520/1974.
In the method, the glass plate suspended in the liquid coolant is rolled by the flow and it is formed certain deformation at the suspending part or it is fallen down. The operation has been not easy and it has been difficult to impart quenching in uniform and to give enough strength disadvantageously.